In recent years, lithium secondary batteries have been widely used as power supplies for downsized electronic devices such as mobile telephones, notebook-size personal computers and the like, and also as power supplies for electric vehicles and for electric power storage, etc. In use for mobile communications as mobile telephones, notebook-size personal computers and the like, the requirements for technical evolution for the capacity, the cycle property and the storage property of lithium secondary batteries to be in electronic devices are being increasing more and more with the increase in the consumption power posed by the technical evolution for multiperformance and multifunction of electronic devices.
In electric vehicles, when the power density of lithium secondary batteries is small, then the number of the necessary batteries must be accordingly increased to attain the intended power; and therefore, in these, it is desired to increase and enhance the capacity and the cycle property of the lithium secondary batteries to be held therein, and in addition, the batteries are further required to maintain the battery characteristics under severe conditions.
To satisfy these requirements, lithium secondary batteries are being much improved for increased capacity and elevated voltage thereof, which, however, could increase the electrochemical reaction inside the battery, therefore causing a problem of gas generation through decomposition of the nonaqueous electrolytic solution in the interface between the positive electrode and the negative electrode therein during charge-discharge cycles or high-temperature storage. The gas generation may bring about other problems of lithium ion movement retardation and cycle property degradation, and even battery body swelling during storage.
As a lithium primary battery, for example, there is known a lithium primary battery comprising manganese dioxide or graphite fluoride as the positive electrode and a lithium metal as the negative electrode, and this is widely used as having a high energy density. It is desired to inhibit the increase in the internal resistance of the battery during long-term storage and to improve the long-term storability thereof at high temperatures.
Recently, further, as a novel power source for electric vehicles or hybrid electric vehicles, electric storage devices have been developed, for example, an electric double layer capacitor using activated carbon or the like as the electrode from the viewpoint of the output density thereof, and a so-called hybrid capacitor comprising a combination of the electric storage principle of a lithium ion secondary battery and that of an electric double layer capacitor (asymmetric capacitor utilizing both the capacity by lithium absorption and release and the electric double layer capacity) from the viewpoint of both the energy density and the output density thereof; and it is desired to improve the battery performance, especially the high-temperature cycle property and the high-temperature storage property of these capacitors.
Patent Reference 1 discloses a lithium secondary battery using a nonaqueous electrolytic solution with a compound such as dimethyl itaconate or the like added thereto, showing the excellent cycle property and storage property of the battery with the compound added thereto, as compared with a case with no addition of the compound thereto.
Patent Reference 2 discloses a lithium secondary battery using a nonaqueous electrolytic solution to which is added a combination of methyl acetate as a saturated linear carboxylate and vinyl acrylate as an unsaturated carboxylate, showing the excellent cycle property of the battery with vinyl acrylate added thereto, as compared with a case with no addition of vinyl acrylate thereto.
Patent Reference 3 discloses a lithium secondary battery comprising spinel-type lithium manganate as the positive electrode active material and using a nonaqueous electrolytic solution with a compound such as vinyl methacrylate, vinyl acetate, ethyl acrylate or the like added thereto, showing the excellent cycle property and storage property of the compound with the compound added thereto, as compared with a case with no addition of the compound thereto.
However, even though the compounds described in Patent References 1 to 3 are added, the cycle property and the storage property are not as yet sufficiently satisfactory.    [Patent Reference 1] JP-A 2007-258101    [Patent Reference 2] JP-A 2007-335170    [Patent Reference 3] JP-A 2000-223154